Fix length checks in X509_cmp_time to avoid out-of-bounds reads.

Also tighten X509_cmp_time to reject more than three fractional
seconds in the time; and to reject trailing garbage after the offset.

CVE-2015-1789

(Imported from upstream's 9bc3665ac9e3c36f7762acd3691e1115d250b030)

Change-Id: I2091b2d1b691c177d58dc7960e2e7eb4c97b1f69
Reviewed-on: https://boringssl-review.googlesource.com/5124
Reviewed-by: Adam Langley <agl@google.com>